Essential parts of such MOS-SC circuits are:
operational amplifiers the respective quiescent current of which is determined by a resistor or by a constant-current source which may be part of a current mirror, PA1 an on-chip clock oscillator for generating a clock signal or an RC clock oscillator whose frequency is determined by a oscillator resistor and an oscillator capacitor, PA1 capacitors connected between a signal input and a signal output, and PA1 switches in the form of transistors, via which the respective capacitors are charged or discharged during operation by the respective operational amplifiers, clocked by the clock signal. PA1 a) The settling time of operational amplifiers must, on the one hand, be short enough that the error caused by the settling time is sufficiently small, for example amounts to 0.1%, on the other hand the settling time must not be so short that the power requirement is greater than necessary and the noise sensitivity is increased owing to the increase in the noise bandwidth. PA1 b) The concrete settling time of a manufactured operational amplifier is determined by the actually realized value of the resistance which defines its quiescent current, or by the actually realized value of the current of the constant-current source; for this the production tolerance in each case lies in the region of 20%. PA1 c) The concrete transconductance of the individual transistors essentially depends on tolerances of the doping of the individual semiconductor regions, on tolerances of the thickness of silicon dioxide layers produced or deposited, that is to say present, outside the gate region, in other words the so-called field oxide, on tolerances of the gate threshold voltage and on tolerances of the channel length; for this the production tolerance lies in the region of 50%. PA1 d) The concrete value of the tolerance of the capacitance of the capacitors usually amounts to 20%. PA1 e) The resistance in the current-conducting state of the transistors which realize the switches, that is to say their so-called respective ON resistance, must, on the one hand, be small enough that the time constant formed by it and the associated capacitor is small enough, and, on the other hand, must not be so small that clock feedthrough and greater leakage effects than necessary occur. PA1 f) The time constants of the individual switch-capacitor elements determine, together with the settling time of the respective operational amplifiers and, furthermore, together with the operating temperature and the concrete value of the operating voltage, the total settling time. In this case, the respective switch-capacitor-operational amplifier units must have settled within a time duration which is determined by the pulses generated by the clock oscillator. In this case, all of the abovementioned tolerances are effective or to be taken into account, which lie in the region of 50 % according to the above explanations. The tolerance of the frequency of an on-chip RC clock oscillator in this case lies in the region of 20% to 30%. PA1 g) Since the above mentioned tolerances of the SC circuit and the last-mentioned tolerance of the clock oscillator are generally not correlated with one another, and are thus added, a tolerance range which is too large to be taken into account results for the abovementioned design from the worst case point of view. This situation can be illustrated by the difference between the period of the clock signal and the required typical value of the settling time of the operational amplifiers, which difference is in this case referred to as margin M and lies in the region of 80% in the worst case. Even if the above mentioned tolerances of the SC circuit and the tolerance of the clock oscillator are correlated with one another, at best a margin of 30% can be achieved. PA1 having at least one operational amplifier, PA1 having an on-chip clock oscillator for generating a clock signal, PA1 having at least one capacitor and PA1 having at least one switch in the form of a transistor, via which the capacitor is charged or discharged during operation by the operational amplifier, clocked by the clock signal.
In the case of MOS-SC circuits having the two abovementioned types of clock oscillators, their frequency and/or their frequency stability are not so critical. Such MOS-SC circuits are, for example, SC analog/digital converters or SC digital/analog converters or the circuit described U.S. application Ser. No. 08/342,218 of Nov. 18, 1994.